The present invention relates generally to cardiac pacers, and more particularly to improvements in such pacers which provide increased noise immunity and reduced power consumption.
Cardiac pacers, which supply amplitude and rate-controlled electrical pulses to a patient's heart to stimulate muscle contraction, have been developed for both internal applications, wherein the pacer is implanted within the patient's body, and external applications, wherein the pacer is externally carried by the patient and rate, pulse amplitude and sensitivity are adjusted as required by the application. External pacers are typically used in emergency situations, where the patient is awaiting implant of a permanent pacer, or where the operation of other pacers is being tested or adjusted. These pacers are typically battery powered for freedom of movement, and must be designed to provide long operating life and a high degree of reliability in often adverse hospital and clinic environments.
Cardiac pacers generally operate in one of three modes: a fixed rate mode, wherein stimulation pulses are produced at a user-selected rate regardless of the occurrence of natural heart pulses; a synchronous mode, wherein stimulation pulses are produced at the end of a predetermined time period following the production of a natural heart pulse; or a demand mode, wherein stimulation pulses are produced only in the absence of a particular heart pulse.
Typically, in the demand mode heart pacers produce a stimulation pulse in the absence of a naturally occurring R-wave signal corresponding to ventricular contraction, which is detected in a sense signal derived from the heart by means of electrically conductive leads physically attached to the heart. The stimulation pulse generated by the pacer is typically applied to the ventricle of the heart by the same leads to induce a desired ventricular contraction.
In the presence of electrical interference in the form of noise or hum on the pacer leads, such as may be induced by power distribution lines or line operated electrical equipment, erroneous operation of a demand mode pacer can occur. The pacer may treat the interference as indicative of a natural heartbeat, and not generate a stimulation pulse even if one is required.
Pacers are often programmed to revert to an alternate fixed rate mode in the presence of electrical noise which prevents the reliable reception of R-wave pulses. Unfortunately, the continuous output pulse production of the pacer in the fixed rate mode undesirably increases battery drain and prevents the heart from naturally beating at its normal rhythm.
One method utilized for reducing the effects of noise and hum is the use of a bandpass filter circuit in the sense amplifier of a pacer to isolate desired R-wave and P-wave signals from undesired interference signals. Typically, such filter circuits in external pacers have bandpass characteristics centered on frequencies from 50 hertz to 150 hertz, and bandwidths of two octaves or more. Consequently, these filter circuits provide little rejection of 60 hertz and 50 hertz power line frequency signals, which are often the principal source of interference, and interference-induced inhibition of portable demand mode pacers continues to be a chronic problem.
It has been proposed that a low pass filter be provided to reject all frequencies above 35 hertz, allowing R-wave energy in the 13 hertz to 35 hertz band to pass and rejecting energy at 50 hertz and 60 hertz power line frequencies. For such a filter to be effective, approximately 25 dB attenuation at 50 and 60 hertz power line frequencies is necessary. A low-pass filter having a corner frequency of 35 hertz and 25 dB attenuation at 60 hertz typically requires at least eight poles. Unfortunately, eight pole LC filters are not practical at power line frequencies because of the size of the inductances involved, and eight pole active RC filters have heretofore required the use of undesirably large plastic dielectric capacitors. The problem is compounded in pacers for European use, which would typically require at least a thirteen pole low pass filter to achieve a corner frequency of 35 hertz and 25 dB attenuation at 50 hertz.
Another proposal is that a passive notch filter, such as a twin-T or bridged-T network, be provided at the appropriate power line frequency. Unfortunately, the notch depth and center frequency in such filters is very sensitive to component variations, such as caused by temperature changes and component aging, making their rejection of power line interference in pacer applications subject to unsatisfactory variations.
It has also been proposed that the analog signal derived from the heart be converted to digital information and then processed using digital techniques to extract the cardiac signal. This would require a complex digital system having high data rates, with attendant high power consumption, making this proposal unsuitable in a battery-powered pacer.
The present invention is directed to a cardiac pacer and sense amplifier wherein power line interference is reliably rejected by means of a high stability active notch filter system synchronously driven by the crystal oscillator of the pacer.
A requirement of battery-powered pacers, whether implanted or portable, is that they have low power consumption to prolong battery life. Previous attempts at reducing power consumption resulted in pacers wherein the pulse rate and R-wave detection threshold were subject to variation with changes in battery voltage, since the high current drain of conventional voltage regulator circuits precluded the use of a precision reference voltage source within the pacer. The present invention is further directed to a low current drain reference voltage source for use in a battery-powered cardiac pacer.
Another requirement of portable battery-powered cardiac pacers is that circuit means be provided for monitoring battery condition. Preferably, such circuit means must be compatible with the requirement of low power consumption, should not unnecessarily complicate the circuitry of the pacer so as to keep production costs at a minimum, and should interrupt operation of the pacer when the battery becomes unusable. The present invention is further directed to a battery monitor circuit which meets these requirements.
A further requirement of a portable demand-mode cardiac pacer is that R-wave signals of either positive or negative polarity be reliably detected for use by the pulse control logic circuitry of the pacer. To this end, such detector circuits must maintain a precise reference threshold above which incoming signals are identified as R-wave signals. Unfortunately, prior art detector circuits having such threshold levels have been undesirably complex. The present invention is further directed to an R-wave detector circuit for a cardiac pacer which is simple in construction and which provides a precise detection threshold above which sensed signals of either polarity are detected as R-wave signals.
Accordingly, it is a general object of the present invention to provide a new and improved cardiac pacer.
It is a more specific object of the present invention to provide a cardiac pacer having improved immunity to power line interference.
It is another specific object of the present invention to provide a cardiac pacer having reduced power consumption for extended battery life.
It is another specific object of the present invention to provide a cardiac pacer having an improved battery condition indicating circuit.
It is another specific object of the present invention to provide a cardiac pacer having improved detection capability of sensed R-wave signals.